How to display customer email within the meta order data box

woocommerce_admin_order_data_after_shipping_address is the hook you’re looking for.

add_action( 'woocommerce_admin_order_data_after_shipping_address', 'my_function' );

function my_function( $order ) {
    echo '<div class="jquery-copy"><input type="text" readonly value="' . esc_attr( $order->get_billing_email() ) . '"></div>';
}

There’s a chance you might want to use $order->get_shipping_email() instead.